1. Field of the Invention
The present invention relates to a manufacturing method of a thin film magnetic recording medium.
2. Description of the Prior Art
A magnetic recording apparatus has been developed to deal with a higher recording density, and a magnetic recording medium superior in read/write characteristics at short wavelengths is demanded. At present, magnetic recording media used mainly are prepared by coating magnetic powders to a substrate, and characteristics of the magnetic media are improved to satisfy the above-mentioned demand. However, the improvement comes near limits on magnetic recording media prepared by coating magnetic powders to a substrate.
A thin film magnetic recording medium is developed to exceed the limits. A thin film magnetic recording medium is prepared by vacuum deposition, sputtering, plating or the like, and it has superior read/write characteristics at short wavelengths. Materials such as Co, Co--Ni, Co--Ni--P, Co--O, Co--Ni--O, Co--Fe--O, Co--Ni--Fe--O, Co--Cr, Co--Ni--Cr and the like are tried for a magnetic layer in a thin film magnetic recording media. As a material for a magnetic tape, a Co--O film and a Co--Ni--O film as a partially oxidized film are considered to be most Suitable, and a deposition tape including a magnetic layer made of Co--Ni--O is already available commercially.
Examples of prior art manufacturing methods of deposition tapes are explained below with reference to FIGS. 1 and 2. FIG. 1 shows an example of a continuous vacuum deposition apparatus for manufacturing a thin film magnetic recording tape. Evaporated atoms adhere obliquely to a substrate running on a cylindrical drum to form a magnetic layer. A substrate 100 made of a polymer material runs along a cylindrical drum 200 in a direction denoted with an arrow 101. Reference numerals 210 and 211 denote a supply roll and a winding roll for the substrate 100. Atoms evaporated from an evaporation source 20 adhere to the substrate to form a magnetic layer. An electron beam evaporation source is preferable for the evaporation source 20, and for example a Co-based alloy as an evaporation material 30 is filled therein. An electron beam evaporation source is used as evaporation materials such as Co having a high melting point at a high evaporation rate. Reference numeral 10 denotes an electron beam displayed schematically with an arrow. A material such as Co having a high melting point evaporates only at a position where the electron beam 10 irradiates, and it hardly evaporates except the position. Shielding plates 50 and 51 prevents for unnecessary evaporated atoms to adhere the substrate and to limit an incident range of evaporated atoms onto the substrate 100. Evaporated atoms passing an aperture defined by the shielding plates 50 and 51 arrive to the substrate 100 to form a magnetic layer. An incident angle of evaporated atoms is defined as an angle of the incident direction of an evaporated atom relative to a normal of the substrate 100. The shielding plate 50 defines an initial incident angle .phi..sub.i of evaporated atoms while the shielding plate 51 defines a final incident angle .phi..sub.f. In an example, .phi..sub.i is 90.degree. and .phi..sub.f is about 30.degree..
Columnar grains grow in a magnetic layer made of Co--O or Co--Ni--O prepared as explained above. As shown in FIG. 2, they grow obliquely and are curved. Therefore, an axis of easy magnetization is oblique relative to a normal of the plane of the magnetic layer.
On the other hand, FIG. 3 shows another example of a continuous vacuum deposition apparatus for manufacturing a thin film perpendicular magnetic recording tape. This example is similar to that shown in FIG. 1, except that the evaporation source 20 is provided vertically below a center of the cylindrical drum 200 for depositing evaporated atoms normally. Shielding plates 50' and 51' define an aperture for film growth or the initial and final incident angles .phi..sub.i and .phi..sub.f of evaporated atoms.
A characteristic of a magnetic layer produced by the apparatus shown in FIG. 3 is strongly uniaxial magnetic anisotropy perpendicular to the substrate. If a magnetic material used is an alloy such as Co--Cr, the anisotropy comes mainly from magnetocrystalline anisotropy. The crystallinity and magnetic characteristics depend strongly on the initial incident angle .phi..sub.i of evaporated atoms to the substrate 100. Therefore, it is necessary to decrease the incident angle to 0.degree. as much as possible. However, productivity on preparing a magnetic layer increases with increase in the length of the aperture. Then, for example .phi..sub.i is set -10.degree. and .phi..sub.f is set 10.degree.. When a magnetic recording tape of Co--O is produced, Co as an evaporation material is filled in the evaporation source 20. The initial and final incident angles are set similarly to a case of Co--Cr alloy.
In Co--Cr and Co--O magnetic layers produced as explained above, columnar grains grow on the substrate 100, and an axis of easy magnetization thereof align generally along a normal of the film plane. However, columnar gains in the magnetic layers are curved as shown in FIG. 4, and this curving is a large factor to deteriorate uniformity of magnetic anisotropy of a magnetic layer.